In general terms, such a lock up clutch includes a coupling element which acts like a piston, together with a drive device. The coupling element is adapted to lie closely against the casing wall opposite to the turbine wheel under the action of the hydraulic pressure, which is appropriately controlled for this purpose, so that the coupling element becomes itself coupled by friction with the casing wall for rotation therewith. The drive device is coupled with the coupling element for rotation therewith so as to allow the latter to be rotatably driven. The drive device is arranged to be coupled for rotation with any drive member whatsoever, connected for rotation with the turbine Wheel. Such a driving member is most commonly the turbine wheel hub.
The present invention is directed more particularly to the case in which, when torsional oscillations are absorbed by a continuous adaptation of the coupling pressure which enables the peaks of the torque characteristic to be damped, the drive device comprises a simple plate or wheel which is circumferentially rigid, as is described in U.S. Pat. No. 4,353,444 issued on Oct. 12 1982.
This plate, or clutch plate, has tongues or strip portions which are resiliently deformable in the axial direction and which lock the clutch plate to the associated coupling element. In practice it is important that, while the design of the clutch plate must be such as to ensure that the coupling element is driven in rotation in a circumferentially rigid manner, the coupling element must at the same time be allowed axially as much freedom of movement as is necessary for it to be brought into close engagement with the casing wall opposite to the turbine wheel and disengaged therefrom.
It is also important that, in spite of its attachment to the clutch plate, the coupling element must be able to react virtually instantaneously to forces applied by the hydraulic pressure to which it is submitted, without risk of jamming on the hub. The coupling element is slidingly carried, sealingly, on the hub in order to perform its function as a piston.
A final important consideration is that, at least in certain applications, the plate must be so designed as to be able to drive the coupling element rigidly, not only when the assembly is operating in the so-called "driving" mode in which the engine tends to drive the vehicle, but also when it is operating in the so-called "reverse" or "regenerative" mode in which it is the vehicle itself which is tending to drive the engine.
In the United States Patent mentioned above, the tongues or strips employed between the clutch plate and the coupling element extend circumferentially in only one direction, which is the direction corresponding to operation of the assembly in the driving mode; and in order to reduce automatically the consequences of any buckling during operation in the reverse mode, means are provided in association with the clutch plate and coupling element, such that circumferential abutting engagement is with a clearance which tends to limit buckling.